Dynamics of Photoinduced Charge Transfer Processes in Iron Carbene Photosensitizers

Abstract: Transition-metal complexes are widely used as photosensitizers and photocatalysts, and in light-emitting devices [Chem. Rev. 117, 10940 (2017)]. We have recently demonstrated first iron complex [FeIII(btz)3]3+ exhibiting room temperature photoluminescence from a charge transfer state and show that it has a 100 ps excited state charge transfer lifetime which is unprecedented for any iron complex [Nature 543, 695 (2017)]. This was characterized as a rare low-spin FeIII d5 complex with emission from a long-lived doublet ligand-to-metal charge transfer (2LMCT) state. Absence of intersystem crossing in this complex avoids significant excited state energy losses as encountered in the prevailing class of d6 transition metal complexes of e.g. FeII and RuII. Most recently, we have also shown [FeII(btz)3]2+ low-spin complex [J. Phys. Chem. Lett. 9, 459 (2018)], a FeII analogue to [FeIII(btz)3]3+. It exhibits strong metal-to-ligand charge transfer (MLCT) absorption bands throughout the visible spectrum, and excitation of these bands gives rise to a 3MLCT state with a 528 ps excited state lifetime in CH3CN solution that is more than one order of magnitude longer compared to the MLCT lifetime of any previously reported FeII complex. Together, these results show that the FeII and FeIII oxidation states of the same Fe(btz)3 complex feature long lived MLCT and LMCT states, respectively. This demonstrates the versatility of iron N-heterocyclic carbene complexes as promising light-harvesters for a broad range of photophysical and photochemical applications, and on-going efforts for further improvements of the excited state dynamics of such systems will be outlined.